1. Field of the Invention
The present invention relates to a process for coating a chromophore containing film onto a polycarbonate or polymethylmethacrylate comprising base. More particularly, the present invention relates to a process for preparing an information recording medium employing a polycarbonate or polymethylmethacrylate comprising base, and a solvent solution for use in the process.
2. Description of the Prior Art
Optical recording methods in which light from the laser is focused upon the surface of a recording medium with sufficient intensity to cause a detectible change in the physical characteristics of the surface material have been proposed. Among these methods is the establishment of an information pattern of pits.
The information recording media which have been used in such optical recording methods involve the writing of information in a thin film of metal or the like formed on a substrate. See, e.g., U.S. Pat. No. 4,238,803. Dyes and pigments, however, have also been increasingly employed in information layers of recording media, often to enhance the sensitivity of the recording layers at the particular wavelength of the laser being used, which results in a much sharper recording and reading of the information. For example, see U.S. Pat. Nos. 4,622,179; 4,614,705; 4,605,607; 4,492,750; 4,529,688; 4,458,004; 4,298,975; 4,241,355; as well as European patent application No. 0188331.
The problems encountered in using chromophores in the information layers of optical media, among others, have generally involved difficult processing or film application problems. The dissolution of such chromophores in a suitable solvent has often been a problem which curtails the commercial expectations of a particular chromophore. Even when a suitable solvent has been found for the dissolution of a particular chromophore, however, problems have arisen with the casting of the chromophore solution onto a substrate. One of the biggest problems is the attacking of the substrate by the solvent. This has particularly been a problem with such suitable substrates as polycarbonate or polymethylmethacrylate comprising substrates. These two substrates are becoming well established as the most preferred substrates for applications involving optical recording media in light of their excellent optical properties and stability.
In general, "attacking the substrate" refers to the solvent changing the properties of the substrate so that it is no longer as useful for its intended purpose. The solvent interferes with the function of the substrate by adversely affecting the integrity of the surface geometry of the substrate either physically or chemically. Generally, the solvent dissolves, swells or otherwise adversely affects the substrate.
Besides the obvious physical damage which can result to the substrate, damage to a polycarbonate or polymethylmethacrylate containing substrate can also be more subtle, yet just as serious. The dissolving or swelling need not be major in order to result in a poorly defined or "graded" interface between the chromophore film and the substrate. It has been found that such a graded interface results in lower reflectivity, as well as a very non-uniform and noisy reflectivity, as the laser beam is focused at the interface. It is desired, therefore, that a well defined, very sharp, interface be achieved, which generally leads to a very high, uniform reflectivity.
It is also believed that unacceptable spectroscopic shifts in the optical properties of the information layer might be observed when a graded interface is created. This, of course, results in a loss of sensitivity since the information layer is therefore less precisely attuned to the laser wavelength. Moreover, chromophores become blended to a certain extent with the substrate through the attack or dissolution of the substrate, which results in a relative loss in sensitivity at the preselected laser wavelength vis-a-vis a pure chromophore layer precisely attuned to that laser wavelength.
Also, features such as holes, grooves, pits or bumps are generally molded into a substrate as preformat and tracking information. These features would be adversely affected by any swelling or partial obliteration resulting from the attack of a solvent. Such attacks cannot be tolerated, particularly in a commercialized process, as the preformatting and tracking information must survive the coating process.
One answer to this problem of the solvent attacking the substrate has been to use a subbing layer or coating over the substrate. Thus, the information layer containing the chromophore is cast onto the subbing layer which protects the substrate from the solvent used in formulating the chromophore solution. However, this involves the costly step of providing a subbing layer. Moreover, subbing layers are often designed to be very thin. Accordingly, pinholes often occur in the subbing layer, thereby allowing an attack of the substrate by the solvent in any event through the pinholes. Total protection of the substrate is therefore not always assured through the use of a subbing layer. As well, there is always the potential of a reaction between the subbing layer and the recording layer if the subbing layer is not carefully selected. The subbing layer can also limit the lifetime of the recording medium if it does not have suitable stability characteristics comparable to the substrate and recording layer. Therefore, the presence of the subbing layer can be detrimental from an archival standpoint if not carefully selected.
The use of a subbing layer does not therefore necessarily provide an acceptable or desirable solution to the problem of solvent attack on a substrate, and particularly a polycarbonate or polymethylmethacrylate comprising substrate. For all of the aforementioned reasons, it would be most beneficial and desirable if a less complicated system was used which eliminated the necessity for a subbing layer.
U.S. Pat. No. 4,639,745 employs what is in effect a subbing layer as protection for the substrate, but refers to it as a recording layer. The patent discloses an optical information medium which comprises a substrate and a first recording layer formed thereon. The first recording layer comprises a light-sensitive material which has been dissolved in a solvent that does not adversely affect the substrate. A second recording layer is then formed over the first recording layer.
In any event, a manufacturing process would be much more attractive if the use of a subbing layer, or a second recording layer, could be avoided altogether, and the chromophore containing solution for forming the active recording layer is cast directly onto the substrate.
Accordingly, it is an objective of the present invention to provide a process for coating a chromophore containing film directly onto a polycarbonate or polymethylmethacrylate comprising base without interfering with the function of the substrate, i.e., so as to maintain the integrity of the surface geometry of the substrate.
More particularly, it is an object of the present invention to provide a process for manufacturing an optical recording medium wherein a chromophore containing solution is cast directly onto a substrate without adversely affecting the substrate.
It is another object of the present invention to provide such a method for manufacturing an optical recording medium, wherein the optical recording medium comprises a polycarbonate or polymethylmethacrylate comprising base.
Still another object of the present invention is provide a unique solvent system for use in coating an aza-annulene chromophore film onto a polycarbonate or polymethylmethacrylate substrate.
Yet another object is to provide a method for safely and efficiently casting an aza-annulene film onto a polycarbonate of polymethylmethacrylate base.
These and other objects, as well as the scope, nature and utilization of the invention, will be apparent to those skilled in the art from the following description and the appended claims.